In recent years, there has been considerable interest in developing approaches to covalently attach poly and oligosaccharides to protein molecules. This approach has been applied in the area of vaccine development, where purified bacterial capsular polysaccharides have been covalently attached to protein molecules (Dick, W. E. et al., 1989). These constructs have been termed conjugate vaccines.
The reason for preparing these constructs is that purified bacterial capsular polysaccharides, which are classified as t-cell independent antigens, can be converted into t-cell-like antigens by covalent attachment to certain protein molecules. Unconjugated polysaccharide vaccines are not capable of eliciting an anamnestic response in man, and the immune response to these antigens can be of limited duration, especially in younger populations. For this reason, the polysaccharide vaccines have not been recommended for usage in infant populations, because of their inherent limited efficacy in this population.
Over the last ten to fifteen years, purified capsular polysaccharide from Haemophilus influenzae type b has been covalently attached to a number of protein molecules, e.g. diphtheria toxoid and tetanus toxoid protein, and these conjugates are known to elicit a t-cell dependent immune response in the infant population. This feature has allowed the development and licensure of effective vaccines against disease caused by the bacterium Haemophilus influenzae type b (Santosham, M., 1993). This approach of preparing conjugate vaccines has also been extended to other capsular polysaccharides, such as those purified from Neisseriae meningitidis and Streptococcus pneumoniae.